1. Field of the Invention
This invention relates to 9 Chromium - 1 Molybdenum, modified steel alloys having superior high temperature properties and weldability, methods for preparing such alloys and articles produced therefrom. In particular, the present invention relates to a method for heat treating such alloys to increase the high-temperature strength thereof and weldability of forgings fabricated therefrom and to high-pressure steam turbine rotors fabricated from such forgings.
2. Description of the Related Art
In a paper published Apr. 7, 1982, authored by P. Patriarca and entitled "Modified 9Cr1Mo Steel Technical Program and Data Package for Use in ASME Section 1 and VIII Design Analysis", the Oak Ridge National Laboratory (ORNL) presented a significant amount of technical data on their new 9 Chromium - 1 Molybdenum, modified (9Cr-1Mo, mod.) alloy. This new ORNL alloy was a modification, of existing 9Cr-1Mo alloys. These new modifications which were made to the chemical composition of 9Cr-1Mo alloys by the ORNL, resulted in a significant increase in the high-temperature properties of the alloy. The new ORNL 9Cr-1Mo, mod. steel alloy is now commercially available as an ASTM standard specification, as set forth in Table 1.
TABLE 1 ______________________________________ Details of commercial ASTM forging specification for ORNL 9Cr--1Mo, mod. ______________________________________ Specification ASTM A-182, F91 Chemistry Element Range (wt %) ______________________________________ C 0.08-0.12 Mn 0.30-0.50 Si 0.2-0.5 P 0.02 max S 0.01 max Cr 8-9.5 Ni 0.4 max Mo 0.85-1.05 Cu 0.1 max V 0.18-0.25 Nb 0.06-0.10 N 0.03-0.07 Al 0.04 max ______________________________________ Tensile and Hardness Requirements Tensile Strength, min. ksi 85 Yield Strength, min. ksi 60 % Elongation, min. 20 Reduction of Area, min. 40 Brinell Hardness Number, max. 248 ______________________________________ Heat Treatment Normalized - 1900 to 2000.degree. F. Tempered - 1350.degree. F. minimum ______________________________________
The principal aim of the ORNL in developing the 9Cr-1Mo, mod. alloy described in Table 1 was to produce an alloy having significant improvement in high-temperature strength, toughness, and corrosion resistance over the prior 21/4 Cr-1Mo pressure vessel steel. The ORNL anticipated that the new 9Cr-1Mo, mod. alloy would find use in the fabrication of boiler tubes, miscellaneous boiler components and for general replacement of 21/4 Cr-1Mo pressure vessel parts.
Conventionally, rotors for high-pressure steam turbines have been fabricated from Chromium-Molybdenum-Vanadium (Cr-Mo-V) alloy forgings. Because of already known design requirements, it is highly desirable that the room temperature yield strength for such Cr-Mo-V rotors should be in the range of from about 85 to about 100 ksi. However, as is seen in Table 1, the ORNL 9Cr-1Mo, mod. alloy steel as set forth in ASTM specification A-182, F91 (Table 1), may have a yield strength of only approximately 60 ksi. Accordingly, at least on the face of it, the ORNL 9Cr-1Mo, mod. alloy steel would not be useful in connection with the fabrication of high-pressure steam turbine rotors. On the other hand, as is well known in the art to which the present invention applies, design engineers continually search for improved materials to upgrade turbine construction and in particular to increase the nominal operating life of the overall turbine installation. The present invention is a result of such search.